I was working on our cabin, at about 4,000 in the Sierra, on one of the hottest days of the year, covering the lower section of the cabin with plywood sheathing. It was a hot, dry job, and i was trying to get it done in one day.

Somewhere near the end of the day, I began to conclude that it was getting cooler, because I wasn't sweating quite so much as I had been earlier in the day. The fact that I considered this a good thing is an indication that my brain was now working with an entirely new set of synapses created by a lack of water in my system.

There was a tiny voice in my brain that said "maybe you aren't sweating because you are completely dehydrated" but I didn't listen to that voice.

After all, I only had one more sheet to install. I measured the wall, and then I marked the plywood. Then I measured the wall a second time, just to make sure. And I checked that measurement again on the plywood. Yep, it looked like it would fit perfectly.

So I carried the plywood over to the wall, and held it in place. It was six inches too long.

What the heck? I measured the wall again---yep, got that right.

Then I measured the plywood.

Ah. I had forgotten to CUT the plywood.

That's when I realized that I really was very dehydrated. Two large glasses of water later, I put on the final sheet of plywood and called it a day. Whew!

And if this had happened on a trail, and I had made a simple mistake like taking the wrong fork in the trail?